First off, download your cloud saves backup from the Extras tab. If you already synced the corrupt save, it might be useless, but best get it now if you didn't right after the power cut.
This game apparently keeps 5 backup saves, so the situation should be fixable. Open File Explorer, then paste %LOCALAPPDATA%\WB Games\Shadow of War\ into the address bar. Backup that entire folder before doing anything.
<Device Name>_ShadowOfWar.sav is the current save, which in your case is corrupt. Delete it.
<user-id>_#_ShadowOfWar.sav are the backup saves. Create a copy of the most recent one and rename it to the current save filename.
Disable cloud saves before starting the game next time. Otherwise, GOG Galaxy might "helpfully" replace your older but working save with the newer corrupt save.
